The conversion efficiency of a catalytic converter may be monitored using a pre-catalyst oxygen sensor (HEGO sensor) positioned upstream from the catalytic converter and a post-catalyst oxygen sensor (catalyst monitor sensor or CMS) positioned downstream from the catalytic converter. One method known for monitoring converter efficiency is to calculate a ratio of CMS transitions or switches to HEGO transitions or switches. An increasing switch ratio is generally indicative of a degrading catalyst.
Catalytic conversion efficiency may vary greatly depending upon the particular operating conditions. To provide an accurate indication of conversion efficiency it is desirable to monitor performance under similar operating conditions over time. To reduce variation of the switch ratio due to factors other than converter degradation, a number of global and local entry conditions must be satisfied before a HEGO switch or a CMS switch is counted. Thus, any switch which occurs when the entry conditions are not met is not counted. To assure a statistically significant sample, a predetermined number of HEGO switches should be counted prior to determination of the switch ratio.
The prior art systems and methods for determining catalyst conversion efficiency based on a switch ratio do not account for the propagation delay or transit time for the exhaust gases to travel from the upstream sensor(s) to the downstream sensor. As such, downstream sensor switches which are attributable to exhaust gases created while entry conditions were satisfied may not be counted if entry conditions are no longer satisfied when the exhaust gases reach the downstream sensor. This affects the accuracy and variation of the switch ratio.
Due to packaging considerations, some engines require separate HEGO sensors for each cylinder bank even though the exhaust from both cylinder banks is combined prior to entering the catalytic converter. Prior art systems and methods may erroneously determine the switch ratio for a converter in these systems due to the cancellation effect of the combined cylinder bank exhausts.